First Minister Arlene Foster is facing fresh calls to resign following new revelations over a bungled heating scheme set to cost Northern Ireland upwards of £400m.

Stormont's opposition parties escalated their criticism of the DUP leader after it emerged her special adviser, Andrew Crawford, has no record of being told of concerns over the botched Renewable Heat Incentive - a 'green scheme' that turned out to be bad for the environment.

As calls mounted for Mrs Foster to appear before the Public Accounts Committee, the Ulster Unionist Party said Mr Crawford should also be considering his position. Mr Crawford's brother, a farmer, was a successful applicant to the RHI as a director of the firm DJC Poultry based in Beragh, Co Tyrone.

Leader Mike Nesbitt's demand came after the DUP stressed Mr Crawford had never met a whistleblower who raised her worries over the scheme directly with Mrs Foster when she was Minister for Enterprise, Trade and Industry (DETI).

A party statement said: "Dr Andrew Crawford was Special Advisor in DETI during the period 2011 to May 2015.

"Dr Crawford did not meet the whistleblower, nor has he any record of these issues having been raised with him at that time."

There is no suggestion that Mr Crawford's brother did anything wrong in applying to the scheme.

However, the SDLP yesterday asked on what scale DUP ministers and MLAs had privately advocated the scheme, and how many DUP members and supporters does Mrs Foster know were beneficiaries.

SDLP leader Colum Eastwood said: "Instead of a story about staggering incompetence, the perception is now growing that there may also have been something more sinister involved.

"The First Minister must not be allowed to deal with this scandal simply on her own terms and she must not be allowed to give answers only to the questions she deems relevant."

Mr Nesbitt said the DUP statement that Mr Crawford had no record of being made aware of the claims was "extraordinary" and raised further questions for Mrs Foster, who has said she passed on the whistleblower's claims to departmental officials.

"It begs the question, why did he not know? Arlene Foster had been contacted by the whistleblower, but she didn't think to tell her own special adviser? It's an absolutely incredible admission and raises further questions for Arlene Foster.

"As a highly paid servant of the public, Dr Crawford should have interrogated the policy, identified the flaw and moved to have it corrected. He clearly failed in his duty.

"The question now is, was the department out of control? On that basis, both the minister responsible, Arlene Foster, and the Spad (special advisor) should consider their positions."

The RHI had encouraged the installation of costly eco-friendly heating systems by paying a tariff over a 20-year period.

But unlike in the rest of the UK, in Northern Ireland no cap was placed on the money that could be claimed.